The shergottites, nakhlites and Chassigny are igneous rocks widely thought to be
samples of the martian crust [e.g., 1]. Discovery of new martian meteorites always
generates interest in the meteoritic community. Interest is heightened if the meteorites are
petrologically unusual, because increased diversity holds promise for increased
understanding of martian petrologic evolution. ALH 84001, originally classified as a
diogenite, has recently been properly classified as a unique orthopyroxenite martian
meteorite [2,3]. Here I report on petrologic and geochemical study of ALH 84001.

Petrography: ALH 84001 is a coarse-grained, cataclastic orthopyroxenite with much
of the original magmatic or metamorphic texture preserved. Generally anhedral
orthopyroxene grains up to 6 mm across exhibit patchy extinction, and contain fractures
and transecting crushed zones of fine-grained anhedral orthopyroxene and chromite
produced by shock. Euhedral to subhedral chromite grains up to 0.5 mm in size are
enclosed in, or are interstitial to, orthopyroxene. Maskelynite, typically a few hundred
µm in size, occurs in interstitial to coarse orthopyroxene and in the crushed
zones. Accessory phases are augite, apatite, pyrite and Mg-Ca-Mn-Fe carbonate. Augite
(~10 µm) and apatite (up to ~300 µm) are interstitial phases. Pyrite
(~10 µm) is associated with interstitial chromite, maskelynite and carbonate, and
in the crushed zones associated with anhedral chromite. Interstitial carbonates (~100
µm), showing fine-scale compositional zoning from Ca-Mn-Fe-rich cores to Mg-
rich rims, are associated with maskelynite and pyrite. Shock damage in these carbonates
demonstrates that they are preterrestrial. Small (~10 µm), homogeneous
carbonates are found in the crushed zones and in fractures, and are post-shock. The
orthopyroxene and chromite grains are relatively homogeneous in composition. Average
mineral compositions are
orthopyroxene - Wo3En69Fs27,
maskelynite - An31Ab63Or6,
augite - Wo42En45Fs13,
carbonate - Cc12Mg58Sd29Rd1.

Geochemistry: The REE pattern for a bulk interior sample of ALH 84001 exhibits a
depletion in LREE relative to HREE, as would be expected for a cumulate
orthopyroxenite, and a negative Eu anomaly (Fig. 1). However, the depletion in LREE is
not as great as expected for a cumulate from a melt with chondritic REE ratios, as shown
by a mean of orthopyroxene clasts from the Johnstown diogenite [4]. The chondritic
normalized La/Lu ratio of ALH 84001 is 0.36, compared to ~0.007 expected for a
cumulate based on opx/melt partition coefficients.

Petrogenesis of ALH 84001: The original protolith had a coarse-grained,
equigranular texture with common 120 triple junctures, indicating slow cooling. The
uniform compositions of orthopyroxenes and chromites similarly indicate slow cooling.
Because ALH 84001 is nearly monomineralic, and texture and mineral compositions
indicate slow cooling, it is likely that ALH 84001 is a cumulate. The interstitial maskelynite
suggests that a trapped melt component is present. Many of the maskelynites are not
stoichiometric plagioclases. Rather, they contain excesses of SiO2 over that required for
stoichiometry. This suggests that the interstitial regions were composed of feldspar plus
silica prior to shock.

The bulk sample contains about 1% maskelynite. Because the trapped melt was probably
basaltic, the amounts of normative plagioclase and pyroxene in this melt were probably
subequal, and therefore the amount of trapped melt likely is ~2%. The REE pattern of
ALH 84001 is consistent with the presence of trapped melt. The higher-than-expected
La/Lu ratio can be explained by inclusion of ~2% of a LREE-enriched melt with a La/Lu
ratio ~4 times CI, roughly half that inferred for the nakhlite parent magma [5]. This
calculated parent melt also has a negative Eu anomaly, with Eu/Sm ~0.6 times CI.

It is possible that the REE pattern of ALH 84001 was affected by infiltration
metasomatism, a process invoked for the nakhlites [6]. The early, zoned carbonates
provide petrographic evidence for fluid fluxes in ALH 84001, and their compositions
suggest that they were formed at ~700°C [7]. This might be taken as evidence for
infiltration metasomatism. However, the early carbonates appear to be discrete grains and
not metasomatic replacements of earlier phases. Regardless, an infiltrating fluid phase, rich
in REE, could cause preferential enrichment in LREE in the cumulate. The parent melt
REE pattern could not then be inferred from bulk REE analyses.

Relationship to Other Martian Meteorites: ALH 84001 has several mineralogic and
petrographic features in common with the nakhlites and Chassigny. The latter are
cumulates with minor interstitial melt [8,9], as is ALH 84001. The nakhlites and Chassigny
contain generally homogeneous pyroxenes [9,10], indicating slow cooling, as inferred for
ALH 84001. The maskelynite compositions of ALH 84001 are within the range of feldspar
compositions in nakhlites and Chassigny [8,9], but more sodic than those in the
shergottites [11-13]. However, many maskelynites in ALH 84001 are similar to some of
the mesostasis compositions of Shergotty and Zagami [12], but with less SiO2. The parent
melt inferred above for ALH 84001 (assuming no infiltration metasomatism) is LREE-
enriched, as were those for the nakhlites and Chassigny [5]. Pyrite is present in
ALH 84001, nakhlites and Chassigny, but not in the shergottites. However, petrogenetic
models for the nakhlites and Chassigny are not compatible with the petrology of
ALH 84001. Estimated parent melts for nakhlites and Chassigny are saturated in olivine
and augite, not orthopyroxene [e.g., 14], and therefore it does not seem likely that
ALH 84001 is directly related to the nakhlites or Chassigny.

Although shergottites contain strongly zoned minerals and textures indicative of rapid
crystallization, ALH 84001 does show several petrologic affinities to the shergottites.
Orthopyroxenes and chromites in ALH 84001 are similar in composition to those of the
megacrysts from EETA 79001 lithology A [11,15] (Fig. 2). Hence, a parent melt with
major element composition like that which formed the megacrysts might be a suitable
parent for ALH 84001. The ALH 84001 orthopyroxene compositions suggest its parent
was slightly more ferroan (Fig. 2). The EETA 79001 megacrysts contain olivine [11],
which is absent in ALH 84001. Olivine is in reaction relation with a megacryst parent melt
composition [16], and therefore, fractional crystallization processes could form an olivine-
free orthopyroxenite from this parent melt. Plagioclase compositions are a severe problem
as the most calcic maskelynite in ALH 84001 (An39) is much more sodic than those in
shergottites [11-13] or than experimental plagioclases [16]. Finally, the calculated
shergottite parent melts are LREE depleted [5], which is incompatible with the trace
element composition of ALH 84001, unless infiltration metasomatism affected the REE
pattern of the protolith.

Thus, ALH 84001 exhibits petrologic affinities to both the nakhlites-Chassigny on one
hand, and the shergottites on the other, as well as differences. The parent melt of
ALH 84001 likely was compositionally distinct from those of the other martian meteorites.